AppFrame - Change media queries for sticky header to older syntax (HDS-5304)

[KristinLBradley]

📌 Summary

If merged, this PR changes media queries in the AppFrame styles related to the sticky positioning of the AppHeader to use level 3 syntax instead of level 4 to be compatible with consumers using compilers not supporting the newer syntax.

• AppFrame standalone example: https://hds-showcase-git-kristin-hds-5304-appheader-me-5ffae2-hashicorp.vercel.app/layouts/app-frame/frameless/demo-full-app-frame-with-app-header-and-app-side-nav
• AppFrame Showcase: https://hds-showcase-git-kristin-hds-5304-appheader-me-5ffae2-hashicorp.vercel.app/layouts/app-frame

🔗 External links

• Jira ticket: HDS-5304

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👀 Component checklist

• Percy was checked for any visual regression
• A changelog entry was added via Changesets if needed (see templates here)

💬 Please consider using conventional comments when reviewing this PR.

📋 PCI review checklist

• If applicable, I've documented a plan to revert these changes if they require more than reverting the pull request.
• If applicable, I've worked with GRC to document the impact of any changes to security controls.
  Examples of changes to controls include access controls, encryption, logging, etc.
• If applicable, I've worked with GRC to ensure compliance due to a significant change to the in-scope PCI environment.
  Examples include changes to operating systems, ports, protocols, services, cryptography-related components, PII processing code, etc.

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Project	Deployment	Preview	Updated (UTC)
hds-showcase	Ready	Preview	Aug 15, 2025 6:05pm
hds-website	Ready	Preview	Aug 15, 2025 6:05pm